Certain methods and systems of scrambling a stream of data are known from WO 95/10906. In the known method, the digital data is divided into packets of N blocks, X(1), X(2), . . . X(N), wherein each block has 2m bits. The sequence of blocks is reversed before the encryption operation into X(N), X(N−1), . . . , X(1). This sequence of blocks is encrypted by the encryption algorithm E in the following manner (where ^ is used to denote an exclusive OR (XOR) operator):Y(1)=E[X(N)^IV]Y(i)=E[X(N−i+1)^Y(i−1)] for i>1 and i≦N. 
The sequence of these encrypted blocks is again reversed, so that the sequence Y(N), Y(N−1), . . . Y(1) is transferred to the receiver.
At the receiver side, the original data blocks are obtained by means of the decryption algorithm D as follows:X(i)=D[Y(N−i+1)^Y(N−i)] for i=1, 2, . . . , N−1X(N)=D[Y(1)]^IV. 
The method used in the known system is indicated as reverse cipher block chaining or RCBC method. It shows the advantage that a buffer storage at the receiver is required for storing two data blocks only.
One example problem of the known method and system is that it requires a buffer at the sender side with the capacity for storing N blocks, in order to implement the reversal of the sequence of blocks. This becomes a problem where there are many senders of encrypted data in a system for data communication, or where a device has to function as both a sender and receiver of data.